2 expand on effects of drawing too much current (see comments)
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I'm designing a system that will power a 2B using an OKI-78SR-5/1.5. It's a switching regulator, pin-for-pin compatible with the classic 7805 linear regulator. I'll be using a 15V PSU (for a motor) and once connected it will stay connected. So after some fairly basic testing I'm just planning on being careful to connect it the right way round. This only does 1.5A, but do you really need more than that on the Pi PCB? If

If you've got further significant 5V power requirements I'd be very tempted to run them separately -- I've crashed a Pi a few times by connecting a not-very-big capacitor to the 5V line using the official 2A PSU, so inrush currents are something to be wary of. They happen fast enough that no reasonable fuse could blow/trip fast enough to prevent a dip in voltage for a big enough surge load so are hard to protect against. Even though I'm guilty of it, I don't recommend plugging anything in while powered up.

I'm not sure what current the PCB traces and headers are rated for, but for large 5V loads you may reach the limit. So there are two reasons to separate the 5V supplies to the Pi and other hardware, even if it's not a situation where isolation is required. One decent 5V supply could even feed both the Pi and the accessories, but I'd run independent cables back to the power supply with capacitors at the power supply output and the Pi input.

I'm designing a system that will power a 2B using an OKI-78SR-5/1.5. It's a switching regulator, pin-for-pin compatible with the classic 7805 linear regulator. I'll be using a 15V PSU (for a motor) and once connected it will stay connected. So after some fairly basic testing I'm just planning on being careful to connect it the right way round. This only does 1.5A, but do you really need more than that on the Pi PCB? If you've got further significant 5V power requirements I'd be very tempted to run them separately -- I've crashed a Pi a few times by connecting a not-very-big capacitor to the 5V line using the official 2A PSU, so inrush currents are something to be wary of.

I'm designing a system that will power a 2B using an OKI-78SR-5/1.5. It's a switching regulator, pin-for-pin compatible with the classic 7805 linear regulator. I'll be using a 15V PSU (for a motor) and once connected it will stay connected. So after some fairly basic testing I'm just planning on being careful to connect it the right way round. This only does 1.5A, but do you really need more than that on the Pi PCB?

If you've got further significant 5V power requirements I'd be very tempted to run them separately -- I've crashed a Pi a few times by connecting a not-very-big capacitor to the 5V line using the official 2A PSU, so inrush currents are something to be wary of. They happen fast enough that no reasonable fuse could blow/trip fast enough to prevent a dip in voltage for a big enough surge load so are hard to protect against. Even though I'm guilty of it, I don't recommend plugging anything in while powered up.

I'm not sure what current the PCB traces and headers are rated for, but for large 5V loads you may reach the limit. So there are two reasons to separate the 5V supplies to the Pi and other hardware, even if it's not a situation where isolation is required. One decent 5V supply could even feed both the Pi and the accessories, but I'd run independent cables back to the power supply with capacitors at the power supply output and the Pi input.

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I'm designing a system that will power a 2B using an OKI-78SR-5/1.5. It's a switching regulator, pin-for-pin compatible with the classic 7805 linear regulator. I'll be using a 15V PSU (for a motor) and once connected it will stay connected. So after some fairly basic testing I'm just planning on being careful to connect it the right way round. This only does 1.5A, but do you really need more than that on the Pi PCB? If you've got further significant 5V power requirements I'd be very tempted to run them separately -- I've crashed a Pi a few times by connecting a not-very-big capacitor to the 5V line using the official 2A PSU, so inrush currents are something to be wary of.